Detection of vibrationally excited methyl formate in W51 e2

Context. Hot cores in molecular clouds, such as Orion KL, Sgr B2, W51 e1/e2, are characterized by the presence of molecules at sufficiently high temperatures to populate their low-frequency vibrationally excited states significantly. Complex organic molecules, such as methyl formate, ethyl cyanide or dimethyl ether, are characterized by a dense spectrum both in the ground state and in the excited states and lines from vibrationally excited states certainly participate to the spectral confusion. Aims. Following a laboratory study of the first torsional excited mode of methyl formate, we search for methyl formate, HCOOCH3, in its first torsionally excited state ($\upsilon_{\rm t}=1$) in the molecular cloud W51 e2. Methods. We performed observations of the molecular cloud W51 e2 in different spectral regions at 1.3, 2, and 3 mm with the IRAM 30 m single dish antenna. Results. Methyl formate in its first torsionally excited state ($\upsilon_{\rm t}=1$ at 131 cm-1) is detected for the first time toward W51 e2. We detect 82 transitions among which 46 are unblended with other species. For a total of 16 A-E pairs in the observed spectrum, 9 are unblended; these 9 pairs are all detected. All transitions from excited methyl formate within the observed spectral range are detected and no strong lines are missing. The column density of the excited state is comparable to that of the ground state. For a source size of 7´´, we find that ${T_{\rm rot}} = 104 \pm 14$ K and $N = 9.4^{+4.0}_{-2.8} \times 10^{16}$ cm-2 for the excited state and ${T_{\rm rot}} = 176 \pm 24$ K and $N = 1.7^{+.2}_{-.2} \times 10^{17}$ cm-2 for the ground state. Lines from ethyl cyanide in its two first excited states ($\upsilon_{\rm t}=1$, torsion mode at 212 cm-1) and ($\upsilon_{\rm b}=1$, CCN in-plane bending mode at 206 cm-1) are also present in the observed spectrum. Blending problems prevent a precise estimate of its abundance, although as for methyl formate, it should be comparable to the value derived for the ground state for which we find ${T_{\rm rot}} = 103 \pm 9$ K and $N = 3.7^{+0.6}_{-0.4} \times 10^{15}$ cm-2 for a 7´´ source size. Conclusions. With regard to the number of lines of excited methyl formate and ethyl cyanide detected in W51 e2, it appears that excited states of large molecules certainly account for a significant number of unidentified lines in spectral survey of molecular clouds.

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